With the increased speed of laser scanning systems the exposure time of a single pixel is exceedingly short. This time is sometimes too short for the desired chemical reaction to take place in the material being exposed. Increasing the power does not solve the problem, as it causes the top layer of the material to become too hot and ablate or decompose, while the material below the surface remains cold.
For example, in a modern laser scanner of the internal drum type used to expose thermal printing plates, dwell times as short as 10 nS are used. This exposure time is much shorter than the thermal time constant of the plate active polymer layer, being about 1 .mu.S for a typical plate. As the exposure time is about 100 times shorter than the thermal time constant, the surface of the polymer has to be heated up to thousands of degrees centigrade to allow the average temperature to be about a hundred degrees (after the heat distributed itself throughout the polymer layer). Such a high peak temperature causes the polymer to decompose or ablate instead of undergoing the desired transformation.
The common solution is to use a multibeam system, as the exposure time of each spot goes up in proportion to the number of beams for a given data rate. Multibeam systems increase the cost of a laser scanner, therefore it is desirable to increase the exposure time of a single beam system. Increasing the exposure time by simply increasing the spot size is not practical due to loss of resolution.
Another object of the invention is to achieve higher utilization of the laser. Thermal imaging systems use high power and expensive IR lasers, typically multiwatt diode-pumped YAG lasers. The present invention enables the use of lasers which are allowed to have poor beam quality in one of the spot dimensions, such as wide area laser diode emitters, which are significantly cheaper than YAG lasers. When multiple light sources are used according to the invention, no intensity matching between the sources is required. This is an advantage over systems using multiple spots in parallel, where intensity match is critical.